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United States Patent |
6,115,203
|
Ho
,   et al.
|
September 5, 2000
|
Efficient drive-level estimation of written-in servo position error
Abstract
The invention relates to a disk drive servo apparatus, and method for
implementing the same, that maintains a transducer on a substantially
non-perturbed path despite the fact that the tracks were written in a
perturbed manner by a servo track writer. The invention utilizes a
non-causal filter to determine servo track writer repetitive run-out
(STW.sub.-- RRO) values for each sector of each track at the drive level.
The STW.sub.-- RRO values are stored, preferably in their respective servo
sectors on the surface of the disk, for use during track following
operations. During read and write operations, the STW.sub.-- RRO values
are retrieved from storage and are used to generate position error signals
necessary to maintain the transducer on the desired path.
Inventors:
|
Ho; Hai T. (Westminster, CO);
Doan; Toan Q. (West Lafayette, CO);
Liikanen; Bruce A. (Berthoud, CO)
|
Assignee:
|
Maxtor Corporation (Longmont, CO)
|
Appl. No.:
|
016552 |
Filed:
|
January 30, 1998 |
Current U.S. Class: |
360/77.04 |
Intern'l Class: |
G11B 005/596 |
Field of Search: |
360/77.04,77.02,77.08,77.11
|
References Cited
U.S. Patent Documents
4412165 | Oct., 1983 | Case et al. | 318/636.
|
5270885 | Dec., 1993 | Satoh et al. | 360/77.
|
5404253 | Apr., 1995 | Painter | 360/77.
|
5608586 | Mar., 1997 | Sri-Jayantha et al. | 360/77.
|
5822147 | Oct., 1998 | Kisaka | 360/77.
|
5825578 | Oct., 1998 | Shrinkle et al. | 360/77.
|
5854722 | Dec., 1998 | Cunningham et al. | 360/77.
|
5886846 | Mar., 1999 | Pham et al. | 360/77.
|
5926338 | Jul., 1999 | Jeon et al. | 360/77.
|
Primary Examiner: Sniezek; Andrew L.
Attorney, Agent or Firm: Sigmond; David M.
Claims
What is claimed is:
1. A method of generating and using compensation values for a disk drive,
the method comprising the steps of:
providing a data storage disk in the disk drive, the disk having a track,
the track having an ideal shape and an actual written shape;
deriving a non-causal impulse response, including
obtaining bode data of the disk drive's error transfer function (S(z));
performing a least square fit to calculate a rational polynomial transfer
function of S(z);
inverting the rational polynomial transfer function of S(z) to obtain the
inverse, S.sub.i (z), thereof; and
performing a partial fraction expansion of S.sub.i (z) to obtain the
non-causal impulse response;
generating the compensation values using the non-causal impulse response;
and
estimating the actual written shape of the track using the compensation
values and the disk drive.
2. The method of claim 1 wherein deriving the non-causal impulse response
includes filtering an intermediate non-causal impulse response to block
unwanted frequencies.
3. The method of claim 1 wherein deriving the non-causal impulse response
includes truncating portions of an intermediate non-causal impulse
response which converge to zero or otherwise settle out.
4. The method of claim 1 wherein the disk drive includes firmware and the
method includes storing the non-causal impulse response in the firmware.
5. The method of claim 1 including convolving the non-causal impulse
response with measured drive-level repetitive run out values to obtain the
compensation values.
6. The method of claim 5 wherein the disk drive includes a transducer
adapted to read information from and write information to the disk and the
method includes adjusting the transducer so that it reads from and writes
to a position relative to a centerline of the ideal shape of the track.
7. The method of claim 6 wherein the compensation values indicate a
positional difference between the ideal shape of the track and the actual
written shape of the track.
8. The method of claim 7 wherein the actual written shape of the track
includes a centerline and the compensation values indicate a positional
difference between the centerline of the ideal shape of the track and the
centerline of the actual written shape of the track.
9. The method of claim 8 including storing the compensation values on the
disk.
10. The method of claim 9 including reading the stored compensation values
from the disk.
11. The method of claim 10 including obtaining an original position error
signal indicating the position error of the transducer from the centerline
of the actual written shape of the track.
12. The method of claim 11 including subtracting one of the compensation
values from the original position error signal to obtain a modified
position error signal for positioning the transducer along the centerline
of the ideal shape of the track.
13. The method of claim 12 including delivering the modified position error
signal to a compensator.
14. The method of claim 13 including generating a compensation signal to
position the transducer along the centerline of the ideal shape of the
track.
15. The method of claim 8 wherein the track includes sectors including
servo regions and data regions, the method further including storing the
compensation values in the servo regions.
16. The method of claim 15 including calculating the compensation values
for one or more designated sectors of the track.
17. The method of claim 15 including calculating the compensation values
for all sectors of the track.
18. The method of claim 8 including storing the compensation values in a
random access memory in the disk drive.
19. The method of claim 5 wherein the repetitive run-out values are each
measured by averaging position error signals over a predetermined number
of revolutions of the disk.
20. The method of claim 1 including calculating, in part, the non-causal
impulse response by computer modeling.
Description
FIELD OF THE INVENTION
The invention relates in general to transducer positioning in a magnetic
data storage system and, more particularly, to compensation for repetitive
run-out (RRO) created by a servo track writer (STW) in a magnetic data
storage system.
BACKGROUND OF THE INVENTION
A disk drive is a data storage device that stores digital data in tracks on
the surface of a data storage disk. Data is read from or written to a
track of the disk using a transducer that is held close to the track while
the disk spins about its center at a substantially constant angular
velocity. To properly locate the transducer near the desired track during
a read or write operation, a closed-loop servo scheme is generally
implemented that uses feedback data read from the disk surface to align
the transducer with the desired track. The servo data is written to the
disk using a servo track writer (STW).
In an ideal disk drive system, the tracks of the data storage disk are
non-perturbed circles situated about the center of the disk. As such, each
of these ideal tracks includes a track centerline that is located at a
known constant radius from the disk center. In an actual system, however,
it is difficult to write non-perturbed circular tracks to the data storage
disk. That is, problems (such as inaccuracies in the STW and disk clamp
slippage) can result in tracks that are written differently from the ideal
non-perturbed circular track shape. Positioning errors created by the
perturbed nature of these tracks are known as written-in repetitive
run-out (STW.sub.-- RRO). The perturbed shape of these tracks complicate
the transducer positioning function during read and write operations
because the servo system needs to continuously reposition the transducer
during track following to keep up with the constantly changing radius of
the track centerline with respect to the center of the spinning disk.
In certain conventional systems, as will be understood by those skilled in
the art, the STW is used to directly measure the STW.sub.-- RRO for each
track of a disk so that compensation values may be generated and used to
position the transducer along an ideal track centerline. In such systems,
the STW must measure the STW.sub.-- RRO of each track of a disk one track
at a time. Because (1) a typical disk drive has four or more disks, (2) a
typical disk contains over 10,000 tracks per inch (TPI) and (3) typical
disk rotation speeds are around 5400 revolutions per minute (RPM), the STW
could be tied-up for several hours in measuring the STW.sub.-- RRO for one
disk drive. The values of the STW.sub.-- RRO for each track (or section of
track) are then stored on the disk for use during transducer positioning.
For an example of a disk drive system that is similar to the above
described system, reference is made to U.S. Pat. No. 4,412,165 to Case et
al. entitled "Sampled Servo Position Control System."
As is well-known in the art, STW's are very expensive and, therefore, only
a limited number of STW's are available at a disk drive manufacturing
facility. Accordingly, by tying-up the STW's for extended periods of time
in measuring the STW.sub.-- RRO for each disk drive, the manufacturing
throughput/efficiency will be dramatically decreased.
Therefore, it would be advantageous if a system were provided for
compensating for the STW.sub.-- RRO without requiring the use of a STW to
determine the STW.sub.-- RRO so that such system could be implemented in a
high volume production environment.
SUMMARY OF THE INVENTION
The invention relates to a disk drive transducer positioning system and
method for implementing the same that is capable of canceling written-in
repetitive run-out in substantially real time. Using the system, the
transducer of the disk drive will follow a substantially non-perturbed
circular path over the disk even though the written track is perturbed as
compared with an ideal track. The system provides a significant
improvement in, at least, track misregistration, write fault performance
and seek settling time over disk drives that do not include the system.
The system is of particular benefit in systems having a relatively high
track density.
In accordance with the invention a disk drive system is disclosed. In one
embodiment, the disk drive system comprises a data storage disk having one
or more tracks. Each of the tracks have an ideal shape and an actual
written shape. The disk drive further includes means for estimating the
actual written shape of the track. The means used does not include a STW.
Rather, the means uses the disk drive containing the data storage disk.
A method of generating compensation values for a disk drive is also
disclosed. One method includes the steps of: (1) providing a data storage
disk in the disk drive, the data storage disk having a track, the track
having an ideal shape and an actual written shape; and, (2) estimating the
actual written shape of the track using the disk drive containing the data
storage disk.
Other objects, features and advantages of the invention will be apparent
from the following specification taken in conjunction with the following
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of a data storage disk illustrating a perturbed data
track that can be compensated for in accordance with the present
invention;
FIG. 2 is a schematic diagram of a servo burst pattern that is used to
position a transducer with respect to a track centerline;
FIG. 3 is a block diagram illustrating the relationship between servo track
writer repetitive run-out (STW.sub.-- RRO) and the position error signal
(PES.sub.-- RRO that is determined for a particular track;
FIG. 4 is a flowchart illustrating a method for determining a non-causal
impulse response that can be used to calculate servo track writer
repetitive run-out (STW.sub.-- RRO) values in accordance with the present
invention;
FIG. 5 is a flowchart illustrating methods in accordance with the present
invention for calculating servo track writer repetitive run-out
(STW.sub.-- RRO) values using a non-causal impulse response and for using
the STW.sub.-- RRO values so calculated to compensate for the perturbed
nature of the data tracks;
FIG. 6 is a block diagram illustrating a servo-loop in accordance with the
present invention for canceling the STW.sub.-- RRO so that a transducer
follows a desired ideal or non-perturbed path; and,
FIG. 7 illustrates, in diagrammatic form, the operation of a non-causal
filter which is used to convolve the PES.sub.-- RRO with the non-causal
impulse response, S.sub.i (k) s.sub.i (k), as set forth in step 66 of FIG.
5.
DETAILED DESCRIPTION
FIG. 1 illustrates a data storage disk 10 that is used to store digital
data in a magnetic disk drive system. The disk 10 is substantially
circular in shape and includes a center point 12 located in the center of
the disk. The disk 10 also includes a plurality of tracks on an upper
surface 14 of the disk 10 for storing the digital data. As discussed
above, each of the tracks is ideally non-perturbed and ideally shares a
common center 12 with the disk 10, such as ideal track 16 illustrated in
FIG. 1. Due to system imperfections, however, actual written tracks on the
disk 10 can be perturbed as compared to ideal tracks, such as non-ideal
track 18 in FIG. 1. Consequently, transducer positioning is not as
accurate on track 18 than it would be on an ideal track. The present
invention provides a system that allows a transducer to follow the path of
an ideal track, such as the path of track 16, using the servo information
written in a non-ideal track, such as track 18. The system of the present
invention therefore approximates a system having an almost perfect servo
track written disk.
As illustrated in FIG. 1, the tracks on the disk 10 are each divided into a
plurality of sectors 22. Each sector 22 is divided into a servo data
portion and a user data portion (not shown). The servo data portion
includes, among other things, information for use by the disk drive in
locating a transducer above a desired track of the disk 10. When a host
computer requests that data be read from or written to a particular
track/sector of the disk 10, the transducer must first be moved to the
track and then must be positioned at a predetermined location with respect
to the centerline of the track before data transfer can take place. For
purposes of illustrating the present invention, it will be assumed that
the transducer should be placed on the track centerline in order to read
from and write to the disk. It should be noted that the invention is not
limited to solely reading and writing when the transducer is placed at its
track centerline.
Referring again to FIG. 1, the disk drive uses the information stored in
the servo data portion of each sector to first locate the desired track
and to then appropriately position the transducer with respect to the
centerline of the desired track. The user data portion of each sector 22
includes data that can be accessed by an external host computer for use in
connection with a central processing unit (CPU) located therein. In
general, the number of sectors per track on the disk is a matter of design
choice. The number may be dictated by, for example, a required servo
update rate for the disk drive.
FIG. 2 illustrates a typical servo pattern 24 stored within the servo
portion of a sector 22 for use in centering a transducer 40 on a desired
track. The servo pattern 24 includes a plurality of servo bursts 26-32
that define the centerlines 34-38 of the tracks of the disk 10. The bursts
26-32 are divided into A bursts 26, 30 and B bursts 28, 32 that are each
approximately a track-width wide and which alternate across the disk
surface. The boundary between an A burst and an adjacent B burst (e.g., A
burst 30 and B burst 28) defines the centerline (e.g., centerline 36) of a
track on the disk. To center the transducer 40 using the A and B bursts,
the transducer 40 is first moved to the desired track during a seek
operation and, once there, is allowed to read the A and B bursts on the
desired track. The signal magnitudes resulting from reading the A and B
bursts are then combined (such as by subtracting the B burst magnitude
from the A burst magnitude) to achieve an error signal, known as the
position error signal (PES), which is indicative of the distance between
the center of the transducer 40 and the centerline of the desired track.
The PES signal is used by the disk drive to change the position of the
transducer 40 to one that is closer to the desired (centered) position.
This centering process is repeated for each successive sector on the track
until the requested read/write operation has been performed in the
appropriate sector 22 of the disk 10. It should be appreciated that other
schemes for storing servo information on the magnetic media (such as
schemes using zones, constant linear density (CLD) recording, split data
fields, and/or hybrid servo) can also be used in accordance with the
present invention.
The A and B bursts 26-32, as well as other servo information, are written
to the surface 14 of the disk 10 using a servo track writer (STW) after
the disk 10 is assembled into the disk drive during the manufacturing
process. It is these A and B bursts which define the location of the
written tracks on the disk 10. That is, on a non-ideal track (such as
track 18 of FIG. 1) the A and B bursts are written such that the
centerline of the track is not smooth, but rather is perturbed. In
conceiving of the present invention, it was appreciated that a transducer
could be made to follow the path of an ideal track by adding an
appropriate offset value to the PES signal in each of the sectors of a
particular track. As illustrated in FIG. 1, the offset amount between the
centerline of the non-ideal track 18 and the path of the ideal track 16 is
different in each sector 22 of the track. The offset values that are used
to modify the PES signal are known as STW.sub.-- RRO values. In accordance
with the present invention, STW.sub.-- RRO values are stored within the
servo portions of each sector 22 of the disk for use in positioning the
transducer 40 on an ideal track path during track following operations.
In one aspect of the present invention, a method and apparatus is provided
for determining the STW.sub.-- RRO values that are later stored in the
servo portions of the disk 10.
The method and apparatus uses a non-causal impulse response to calculate
the STW.sub.-- RRO values from measured PES values of the drive. As
described above, once determined, the STW.sub.-- RRO values are then
stored on the disk surface for use during track following operations.
If the transducer 40 is to follow a perturbed path, such as that of
non-ideal track 18, the position of the transducer 40 must be constantly
adjusted as the disk 10 rotates. Therefore, when performing conventional
track following on a non-ideal track, adjustments are constantly being
made to the transducer position to keep it centered on the track. The
transducer position is adjusted, as described above, by calculating a PES
signal for each sector 22 of the track and using the PES signal to create
a control signal for a movement means (such as a voice coil motor) to move
the transducer 40 an appropriate amount in each sector. Because the
transducer position is continuously being adjusted, perfect or near
perfect registration between the transducer center and the center of the
track is rarely achieved. This can create problems such as high track
misregistration values.
In conceiving of the present invention, it was appreciated that the PES
values that are calculated for each sector of a given track (which
henceforth will be referred to as PES.sub.-- RRO values) are related to
the STW.sub.-- RRO values for the track by a predetermined transfer
function S(z), as illustrated in FIG. 3. The transfer function, in
general, describes how the servo control system reacts to and follows the
perturbed track 18. That is, STW.sub.-- RRO is the stimulus and PES.sub.--
RRO is the response. To determine the STW.sub.-- RRO values using the
measured PES.sub.-- RRO values, therefore, one needs to find the inverse
transfer function S.sup.-1 (z) and to apply the PES.sub.-- RRO values
thereto. However, the inverse transfer function S.sup.-1 (z) is generally
unstable due to the action of certain elements within the servo control
loop, such as the zero-order hold unit. Thus, conventional methods for
determining the impulse response of the inverse transfer function S.sup.-1
(z) often result in divergent behavior.
In accordance with the present invention, it was determined that the
PES.sub.-- RRO data could be processed in non-real time using a non-causal
impulse response of the inverse transfer function. A non-causal system is
a system that has an impulse response that is nonzero for negative time.
That is, the system is capable of producing a response before any
excitation is applied to the input of the system. As such, non-causal
systems are generally regarded as non-physically-realizable systems.
However, as illustrated by the present invention, non-causal impulse
responses can be utilized as analysis tools to perform non-real time
calculations.
FIG. 4 is a flowchart illustrating a method for determining the non-causal
impulse response of the inverse error transfer function of a servo-loop in
accordance with the present invention. It should be noted that the steps
outlined in FIG. 4 are generally performed off-line by an engineer and are
stored into the disk drive system's firmware.
In step 50, a command is sent to the disk drive to obtain bode data of S(z)
(i.e., poles and zeros), where S(z) represents the error transfer function
of the closed-loop servo system. The bode data is obtained in a
conventional manner by either computer modeling or by direct measurement
using appropriate instrumentation, as is well-known to those in the art.
Next, in step 52, a rational polynomial transfer function of S(z) is
obtained using the bode information by performing a least square fit of
either the computer-modeled data or the measured data. In the case of the
present invention, it is preferred that the rational polynomial transfer
function be obtained by performing a least square fit on the
computer-modeled data. One computer program which is capable of performing
the requisite computer modeling is commercially known as Matlab (although
other similar programs may be used). The rational polynomial transfer
function of S(z) can be described by the following equation:
##EQU1##
where n.sub.0 . . . n.sub.N and d.sub.0 . . . d.sub.D are real and
constant coefficients of the transfer function, N and D are integers and
represent the order of the transfer function, and z is the Z-transform
complex variable. The values of the coefficients, N and D, depend on the
nature of the servo system. This form of transfer function is well-known
to those in the art.
The inverse transfer function, S.sub.i (z), is then determined by swapping
the numerator and denominator of the rational polynomial transfer function
(step 54). The resulting equation is:
##EQU2##
A partial fraction expansion of the inverse transfer function, Si(z), is
then performed (step 56) which yields the following equation:
##EQU3##
where r.sub.1 . . . r.sub.m and p.sub.1 . . . p.sub.m are constant
real-valued numbers, and m is a constant integer. For the present
invention, it is preferred that the partial fraction expansion of S.sub.i
(z) also be performed using the Matlab computer program (again, other
programs may be used).
The partial fraction expansion of the inverse transfer function is then
used to compute the non-causal impulse response S.sub.io (k) (step 58),
where k is the sector time index used in the time domain. The unstable
poles of the non-causal impulse response represent the negative time
portion of the response while the stable poles of the non-causal impulse
response represent the positive time portion of the response. The
non-causal impulse response can be represented by the following equation:
##EQU4##
where
.alpha..sub.i (k)=r.sub.i p.sub.i.sup.k,k.gtoreq.0 if .vertline.p.sub.i
.vertline.<1
and
.alpha.i(k)=-r.sub.i p.sub.i.sup.k,k<0 if .vertline.p.sub.i
.vertline..gtoreq.1
In the above equations, .alpha..sub.i 's represent individual constituent
impulse responses that are summed together to form S.sub.io (k).
A post filtering step is then performed on S.sub.io (k) so that the
non-causal impulse response can be refined by blocking specific unwanted
frequencies (step 59). In the preferred embodiment, a non-causal high pass
filter is combined with S.sub.io (k) to produce a final response, S.sub.i
(k). Finally, the non-causal impulse response is truncated and saved in
the disk drive's firmware (step 60) for later use in determining the
STW.sub.-- RRO values. More specifically, the portions of S.sub.i (k)
which converge to zero or otherwise settle-out are truncated. The
truncation is performed so that the computational burden, when determining
the STW.sub.-- RRO values, is minimized.
As mentioned above, it should be noted that all of the steps performed in
connection with FIG. 4 (to derive the non-causal impulse response) are
generally performed by an engineer prior to the factory calibration
process described below. The derived non-causal impulse response is then
stored in the disk drive's firmware.
FIG. 5 is a flowchart illustrating a method for determining STW.sub.-- RRO
values using the non-causal impulse response stored in the firmware (as
described in connection with FIG. 4) and measured PES data (to be
described in connection with FIG. 5). The steps outlined on the top half
of FIG. 5 are performed during the factory calibration process, while the
steps on the bottom half of FIG. 5 are performed during regular use of the
disk drive.
During factory calibration of a disk drive, a seek operation is performed
(step 62) to move the transducer to a predetermined track of the disk 10.
Once on the desired track, the transducer reads the servo data in the
servo portions of the sectors of the track for multiple revolutions of the
disk 10.
A calibration controller then calculates average PES values for each sector
of the track for a predetermined number of revolutions of the disk (step
64), which is preferably five or more. The average PES values represent
the PES.sub.-- RRO of the disk. PES averages are used so that the
repetitive written-in track profile can be distinguished from the
non-repetitive portion of it (i.e., random noise), as will be understood
by those skilled in the art.
The STW.sub.-- RRO values are then calculated by convolving the filtered
PES.sub.-- RRO values with the non-causal impulse response, S.sub.i (k),
stored in the disk drive's firmware (step 66), to be described in further
detail in connection with FIG. 7. The filtered STW.sub.-- RRO values are
then stored in the appropriate servo sectors on the disk surface (step
68).
As described above, determination of the STW.sub.-- RRO values is performed
at the drive level rather than by using a STW. This increases the accuracy
of the determination considerably as drive level effects, such as PES
channel variations, are accounted for. Furthermore, by determining the
STW.sub.-- RRO at the drive level, valuable STW time is not consumed,
thereby avoiding decreases in manufacturing throughput and efficiency (as
will be understood by those skilled in the art).
During normal disk drive operation, the transducer reads the STW.sub.-- RRO
value stored in each servo sector of a desired track (step 70). The
STW.sub.-- RRO value is then used to modify the calculated PES value to
cancel the offset between the non-ideal track and the desired (ideal)
transducer path (step 72). In the preferred embodiment of the invention,
the STW.sub.-- RRO value is subtracted from the calculated PES value to
obtain a modified PES (PES.sub.m). The modified PES value is then applied
to the track following compensator where it is converted to a voice coil
motor control signal (step 74).
FIG. 6 is a block diagram illustrating the cancellation of STW.sub.-- RRO
in a disk drive in one embodiment of the present invention. The
VCM/actuator/transducer assembly 80 outputs a signal that is indicative of
transducer position. The signal is generated by reading servo information
within a servo sector on the surface of the disk. For example, the servo
signal can include A/B burst information. The position signal is processed
by a decoder/demodulation unit 82 to create an original PES signal,
PES(k), that indicates the distance of the transducer from a predetermined
track centerline.
The VCM/actuator/transducer assembly 80 also outputs a signal that is
indicative of the STW.sub.-- RRO values. The signal is generated by
reading information stored on the surface of the disk which corresponds to
the STW.sub.-- RRO value for that particular sector. The signal is then
processed by the decoder/demodulation unit 82 to obtain the stored value
of the servo track writer repetitive run-out (STW.sub.-- RRO(k)).
Because the predetermined track centerline is offset from the desired
(ideal) transducer path, the original PES signal (PES(k)) is delivered to
the subtractor 84 which subtracts the STW.sub.-- RRO value from the
original PES signal to create a modified PES signal (PES.sub.m (k)). The
modified PES signal is delivered to a compensator 86 which converts the
modified PES signal into a control signal for the VCM. In response to the
control signal, the VCM keeps the transducer substantially centered above
the desired non-perturbed transducer path. Because the transducer is
maintained on a substantially non-perturbed path, very little physical
adjustment of transducer position is required from servo sector to servo
sector, and track misregistration is reduced.
In the preferred embodiment, the STW.sub.-- RRO value is read by the
transducer from the same servo sector that provided the position signal.
It should be appreciated, however, that the STW.sub.-- RRO value may be
stored in other locations within the disk drive, such as in a random
access memory (RAM) or the like.
FIG. 7 illustrates, in diagrammatic form, the operation of a non-causal
filter which is us ed to convolve the PES.sub.-- RRO with the non-causal
impulse response, S.sub.i (k), as set forth in step 66 of FIG. 5. As shown
in FIG. 7, PES.sub.-- RRO(k) is convolved with the non-causal impulse
response resulting in the cancellation values (STW.sub.-- RRO(k)). The
STW.sub.-- RRO(k) values computed by the non-causal filter can be
described as follows:
##EQU5##
where l+q+1 is the length of the filter or the number of delay taps; k is
the sector time index; and, n is a dummy variable.
Although the present invention has been described in conjunction with its
preferred embodiment, it is to be understood that modifications and
variations may be resorted to without departing from the spirit and scope
of the invention as those skilled in the art readily understand. Such
modifications and variations are considered to be within the purview and
scope of the invention and the appended claims.
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